Frequency measurement works by enabling the data capture engine
for a pre-programmed number of signal cycles. At the end of this
measurement period, the capture engine is frozen.

Instead of retrieving captured data from the sample RAM, the
sample address counter itself is
used to determine the duration of the measurement interval and
therefore the frequency of the input signal.

There are five registers used to program trace mode 8:

R3

Sample Pre-load (Low Byte)

Spock Counter/RAM address (low byte).

R4

Sample Pre-load (High Byte)

Spock Counter/RAM address (high byte).

R14

Input/Attenuation

Alt/Chop channel input/attenuation settings.

R21

Frequency Timer Pre-Load

Number of signal cycles to count.

R22

Frequency Pre-scale

Input frequency pre-scale divider.

and two optionally used to read the measurement duration:

R9

Counter Capture (Low Byte)

Spock Counter/RAM address capture (low byte).

R10

Counter Capture (High Byte)

Spock Counter/RAM address capture (high byte).

As with all other trace modes, the counter value is turned
automatically on completion of the trace command so R9 and
R10 may not need to be read.